Scanned beam head-up displays (HUDs) and other scanned beam displays which can include direct view panel displays that allow the viewer's eye to move freely within a defined volume known as the viewing eyebox have a higher requirement for uniformity than head mounted displays (HMDs), especially color uniformity. In some display systems equipped with an exit pupil expander (EPE), it is possible for the eye to be placed in locations such that the beamlet patterns emanating from various EPE field points by the beam interacting with the periodic array of the EPE may not overlap. In such instances, the appearance to the eye is that there is a magnified portion of the beamlet pattern illuminated across the full field of view (FOV), where the amount of apparent magnification depends on distance of eye away from the ideal viewing plane, where all of these beamlet patterns do cross or overlap, and beamlet density which can be defined in terms of angular resolution. When viewing at the ideal plane, the magnification of the backlit pattern is apparently infinite, and a specific intensity level appears across the full FOV, thus no tiling patterns appear to be backlit across the field of view. The intensity level of the full FOV may change according to the beamlet pattern uniformity convolved with eye-pupil size, when the eye moves in an x-y direction.
The typical solution to this problem has been to tile beamlet profiles in the eyebox such that the uniformity is flattened within the eyebox beamlet pattern. To accomplish this, a top hat converter lens may be utilized to obtain a flat top intensity profile from a Gaussian beam laser source. The top hat output then may be clipped to the proper shape to complement the periodic EPE array pitch and layout. When using the proper focusing numerical aperture (NA), the result is that the input beam solid cone will have the same angular content to fully fill the solid angle between diffraction orders of the beamlet pattern, thus achieving a Fill Factor of 1. For a typical scanned beam system, use of any Fill Factor greater than 1 results in a condition that induces Moiré across the FOV wherein sub-cell illumination by an undersized spot does not fully fill all exit angles into the eyebox, and aliasing effects between the raster line spacing and EPE pitch result in a Moiré effect. Resulting uniformity can be satisfactory if all beamlet controls have been properly designed such as beamlet profile, Fill Factor, and/or shape used in conjunction with an EPE capable of forming a uniform exit pattern or diffraction envelope. However, this approach entails tedious alignments and elements such as circularizers, top hat converter lenses, and/or hex apertures with clocking requirements, as well as the expected relationships between spot character and EPE pitch and layout. Further, the use of a focused top hat forms a sinc-like spot that causes loss of contrast in the mid-spatial frequencies of the display due to a ramp-like modulation transfer function (MTF) response.
As a result, light sources for EPE equipped scanned beam systems specifying a uniform eyebox volume have been designed specifically to form cone NAs limited within the solid cone representing a Fill Factor of 1 in order to achieve sufficient uniformity within the eyebox volume while at the same time avoiding Moirés, which manifests itself by apparent fringes within the FOV. Attempts have been made in the past hoping to capitalize on use of Gaussian beamlet profiles by overlapping the Gaussian beamlets near full-width half-maximum (FWHM), however such attempts have always resulted in a condition of sub-cell illumination such that the spot did not fully fill the lenslet or diffractive optical element (DOE) cell. Under such a condition, the exit pupil pattern at the eyebox changes in profile producing hot spots in intensity that depend on the x-y position of the undersized spot within the lenslet or cell. Since neighboring beamlets have differing phase, constructive and destructive interference occurs such that the skirts of the Gaussian either form constructive peaks or extinctions. The dependence of such chances on spot position implies that the eye will see different intensities emanating from different pixel locations due to aliasing between the EPE pitch and the raster line spacing.
When the NA is set such that the spot is equal to or larger than the EPE pitch, and/or cell size, spot location dependency of the eyebox diffraction envelope pattern can be avoided, thus Moiré can be avoided. However when the NA is set such that the spot is larger than a cell, gapping forms between beamlets within the eyebox, and although the diffraction envelope is solid, uniformity within the eyebox suffers, thereby causing apparent tiling artifacts.
It will be appreciated that for simplicity and/or clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, if considered appropriate, reference numerals have been repeated among the figures to indicate corresponding and/or analogous elements.